1. Field of the Invention
The present invention relates to an exposure apparatus and an exposure method.
2. Description of the Related Art
During a photolithography process, which is one of the manufacturing processes implemented to manufacture semiconductor devices, a reduction projection exposure apparatus (stepper) is utilized. In keeping with the miniaturization of semiconductor devices achieved in recent years, rigorous efforts are made to reduce the wavelength of the exposing light and to achieve a higher NA (numerical aperture) in the stepper.
The relationship between the resolving power R of the stepper and the depth of focus (DOF) is expressed through a Rayleigh formulae presented below.
R=k1*xcex/NA 
DOF=k2*xcex/(NA)2 
As these formulae clearly indicate, the focal depth DOF becomes reduced if the wavelength xcex of the exposing light is reduced or the numerical aperture NA is increased to improve the resolving power R. Since the focal depth DOF of the stepper is now achieved in the same order as that of the surface level variation of the semiconductor device, it is necessary to take measures with regard to the defocusing of the exposing light manifesting, in particular, in an area that is not level (an indented area or a projected area), in order to maintain a specific degree of dimensional accuracy for the transfer pattern.
The so-called leveling technology achieved by moving the wafer vertically in correspondence to the surface level variation or even by tilting the exposure surface in some cases during the exposure operation is normally adopted to address the problem discussed above. A surface level variation may be detected by a focus sensor provided in the stepper to measure the distance between the surface of the semiconductor device and a reference position (e.g., the exposing light source) and it may then be expressed with a three-dimensional coordinate system.
In addition, an exposure method achieved by dividing the entire wafer into a plurality of unit areas (hereafter referred to as xe2x80x9cshotsxe2x80x9d) and irradiating slit light on each shot while scanning the slit light is usually adopted in the exposure process in recent years. The range over which the slit light is irradiated is normally approximately {fraction (1/10)} of the entire shot and, by scanning this light, the entire shot becomes exposed.
Adoption of such a scanning method in the exposure process may prove problematic, particularly if non-level portions attributable to the circuit structure are systematically present within the individual shots. If, during the exposure operation, the focal length is adjusted at such a non-level portion, which often manifests a drastic difference in the height relative to the surrounding area, the likelihood of the exposing light becoming defocused over a wide range at the surrounding area increases.
Since the results of measurement performed by the focus sensor are directly utilized in the wafer leveling operation during the scanning exposure process in the related art, a defocus area is formed over a wide range if the measurement point at which the focus sensor performs the measurement hits upon a non-level position, which will result in poorer dimensional accuracy of the transfer pattern.
An object of the present invention, which has been completed by addressing the problems discussed above, is to provide an exposure apparatus and an exposure method that make it possible to minimize the range over which the exposing light is defocused even when an non-level portion is included within each shot.
In order to achieve the object described above, in a first aspect of the present invention, an exposure method achieved by sequentially exposing a plurality of unit areas set in advance at the surface of a wafer is provided.
A first feature that characterizes the exposure method according to the present invention is that a pre-exposure process is implemented prior to exposure processing implemented on individual unit areas. During this pre-exposure process, one or a plurality of measurement target unit areas are selected from the unit areas and a xe2x80x9cvirtual surfacexe2x80x9d and xe2x80x9cadjustment valuesxe2x80x9d are calculated in correspondence to the three-dimensional coordinates indicating the position of the surface in each measurement target unit area.
In a first exposure method according to the present invention, one of the plurality of unit areas is set as a measurement target unit area in a first pre-exposure step, the three-dimensional coordinates of a plurality of measurement points at the surface in the measurement target unit area are determined in a second pre-exposure step and a virtual surface approximating the surface in the measurement target unit area is ascertained by using the three-dimensional coordinates of each measurement point in a third pre-exposure step. It is desirable to calculate the virtual surface through, for instance, the methods of least squares.
In a fourth pre-exposure step implemented next, the surface in the measurement target unit area is scanned by a plurality of sensors and the extents to which the individual scanning areas within the measurement target unit area scanned by the sensors are deviation from the virtual surface along a direction in which a specific coordinate axis extends are determined. The extents of the positional deviation manifesting at the respective scanning areas are stored in memory as adjustment values for the individual sensors, each corresponding to one of the scanning areas at, for instance, the controller of the exposure apparatus. It is to be noted that if a Z axis represents the optical axis of the exposing light, the extents of positional deviation can be expressed as a difference in the Z coordinate value.
When these pre-exposure steps are completed, a sensor selection step is implemented. In the first exposure method according to the present invention, at least two sensors among a plurality of sensors are selected. In a second exposure method according to the present invention, at least two sensors with their adjustment values having the smallest absolute values are selected from a plurality of sensors.
According to the present invention, an exposure unit area setting step is implemented. In this step, one of the plurality of unit areas is set as the exposure unit area to undergo the exposure processing. This step may be implemented any time prior to the exposure step. For instance, a plurality of unit areas may be set on the wafer, all the unit areas may be set as an exposure unit areas and the order in which they are to be exposed may be specified before the pre-exposure process.
In the second exposure method according to the present invention, a virtual surface and adjustment values are obtained through steps which are different from those implemented in the first exposure method according to the present invention. Namely, one of the plurality of unit areas is set as a measurement target unit area in a first pre-exposure step, and then in a second pre-exposure step, the surface in the measurement target unit area is scanned by a plurality of sensors and the three-dimensional coordinates of the individual scanning areas within the measurement target unit area having been scanned by the sensors are ascertained. In a third pre-exposure step, a virtual surface approximating the surface in the measurement target unit area is ascertained by using the three-dimensional coordinates of the scanning areas.
In a fourth pre-exposure step implemented next, the extents to which the individual scanning areas within the measurement target unit area having been scanned by the sensors in the second pre-exposure step are deviation relative to the virtual surface along a direction in which a specific coordinate axis extends are determined. The extents of the positional deviation manifested at the scanning areas are stored in memory as adjustment values for the individual sensors each corresponding to one of the scanning areas.
In addition, in a third exposure method according to the present invention, a virtual surface and adjustment values are obtained through steps which are different from those implemented in the first and second exposure methods according to the present invention. Namely, in a first pre-exposure step, one of the plurality of unit areas is set as a measurement target unit area, in a second pre-exposure step, the three-dimensional coordinates of a plurality of measurement points at the surface in the measurement target unit area are ascertained and in a third pre-exposure step, a virtual surface approximating the surface in the measurement target unit area is ascertained by using the three-dimensional coordinates of the individual measurement points.
In a fourth pre-exposure step implemented next, the positions of at least two sensors relative to the measurement target unit area are adjusted. When three or more sensors are present, too, it is crucial that the positions of the two sensors determined to be necessary for leveling the wafer during the exposure step be adjusted. It is to be noted that a positional adjustment may be implemented for three or more sensors and no problem will arise during the subsequent exposure process from performing such a positional adjustment. The positions of the two or more sensors are adjusted so that when the two or more sensors scan the surface in the measurement target unit area, the extents of positional deviation between the individual scanning areas scanned by the sensors and the virtual surface along the direction in which the specific coordinate axis extends are minimized. Then, in a fifth pre-exposure step, the extents of the positional deviation of the scanning areas scanned by the two or more sensors having undergone the positional adjustment are stored in memory as adjustment values for the corresponding sensors.
As described above, in the first, second and third exposure methods according to the present invention, the xe2x80x9cadjustment valuesxe2x80x9d are obtained through the pre-exposure steps. In fourth and fifth exposure methods according to the present invention, on the other hand, a plurality of measurement target unit areas are selected from a plurality of unit areas and are set, and the second pre-exposure stepxcx9cfourth pre-exposure step are implemented on each of the measurement target unit areas to obtain xe2x80x9cadjustment valuesxe2x80x9d for the individual measurement target unit areas. Then, in a fifth pre-exposure step, xe2x80x9caverage adjustment valuesxe2x80x9d are calculated based upon the adjustment values. Since an adjustment value is used for the wafer leveling control in the exposure step, the accuracy of the adjustment value affects the leveling accuracy. For this reason, the use of the xe2x80x9caverage adjustment valuesxe2x80x9d, which are obtained by measuring a plurality of measurement target unit areas and contain a small measurement error, for the wafer leveling control improves the leveling accuracy.
A second feature that characterizes the exposure method according to the present invention is that an exposure step during which the wafer is leveled by using an xe2x80x9cadjustment valuexe2x80x9d obtained through the pre-exposure steps is implemented.
In the first, second, fourth and fifth exposure methods according to the present invention, the three-dimensional coordinates at the surface of an exposure unit area are measured by some of or all of at least two sensors selected in the sensor selection step. In the third exposure method according to the present invention, the three-dimensional coordinates at the surface of an exposure unit area are ascertained through measurement by some of or all of at least two sensors the positions of which have been adjusted in the fourth pre-exposure step. In any one of these exposure methods, by using the individual adjustment values in processing the results of the measurement by the corresponding sensors (e.g., by subtracting the adjustment values), adjustment results are obtained.
While wafer leveling is implemented in order to reduce the extent to which exposing light becomes defocused when the exposing light is irradiated on the exposure unit area, the presence of a raised area may cause the wafer to be tilted to an excessive degree if the results of the measurement performed by the sensors are directly used in the leveling operation, to result in a wider defocus range. However, the wafer leveling operation is performed using the adjustment results achieved based upon the sensor measurement results and the adjustment values. Since the adjustment values are obtained in relation to the virtual surface, the wafer is leveled in conformance to the virtual surface obtained through the calculation rather than the state of the actual surface of the exposure unit area during the exposure operation. As a result, even if there is a raised area in the exposure unit area, the wafer is not allowed to be tilted to an excessive degree, thereby minimizing the extents of defocusing of the exposing light.
According to the present invention, even if it is not possible to measure the three-dimensional coordinates at the surface of the exposure unit area with two or more sensors simultaneously, the wafer is leveled in the exposure step based upon the adjustment results achieved in conformance to the measurement results obtained by the sensor(s) capable of measuring the three-dimensional coordinates at the surface of the exposure unit area alone. Thus, the extents of defocusing of the a exposing light in the exposure unit area is reduced. In addition, it is desirable to select two or more sensors that are capable of measuring the three-dimensional coordinates the of the surface of the exposure unit area at all times while exposing light is irradiated on the exposure unit area during the exposure step and are set over a largest distance from each other in the sensor selection step. The selection of such sensors achieves stabilization of the wafer leveling operation performed during the exposure processing.
According to the present invention, a calculation-exempt area is set in a measurement target unit area. By setting the calculation-exempt area so as to contain a raised portion present in the unit area, it is possible to almost completely match the virtual surface ascertained through the pre-exposure steps with the actual surface of the measurement target unit area (excluding the calculation-exempt area) and an adjustment value close to zero is obtained. If the adjustment value is small, the calculation error and the measurement error contained therein are also reduced, and ultimately, the accuracy of the wafer leveling control implemented in the exposure step improves.
It is desirable to first ascertain a temporary virtual surface, compare the temporary virtual surface to the values obtained through actual measurement and obtain a final virtual surface by excluding a measurement point manifesting a large deviation during the pre-exposure steps. The virtual surface thus obtained almost completely matches the actual surface of the measurement target unit area and thus, an adjustment value close to zero is obtained. If the adjustment value is small, the calculation error and the measurement error contained therein are reduced and, as a result, the accuracy of the wafer leveling control implemented in the exposure step improves. By adopting this method, the need to set a calculation-exempt area is eliminated.
When exposing a plurality of wafers, the pre-exposure steps may be implemented on one wafer at a time, or the pre-exposure steps may be implemented in units of two or more wafers at a time or over specific time intervals. While the first method makes it possible to adjust for any inconsistency manifesting among manufactured wafers, a reduction in the length of time required to implement the exposure processing on the entire wafer lot is achieved by adopting the latter method.
In a second aspect of the present invention, an exposure apparatus for exposing a wafer is provided. This exposure apparatus comprises one or a plurality of sensors capable of Determining three-dimensional coordinates of a plurality of measurement points at the surface of the wafer and a controller capable of ascertaining a virtual surface approximating the surface of the wafer by using the three-dimensional coordinates of the individual measurement points. This structure enables a wafer leveling operation to be performed in conformance to the virtual surface when exposing the wafer.
When one or a plurality of sensors are capable of scanning the wafer surface, the three-dimensional coordinates obtained at a greater number of measurement points on the wafer can be ascertained with a high degree of efficiency. If the measurement is performed at a greater number of points, the reliability of the virtual surface which is ascertained through calculation (e.g., through the method of least squares) performed in conformance to the measurement results improves.
The controller determines the extents of positional deviation of the individual scanning areas on the wafer scanned by the sensors relative to the virtual surface along a direction in which a specific coordinate axis extends and stores in memory the extents of the positional deviation of the individual scanning areas as adjustment values for the sensors each corresponding to one of the scanning areas. Then, when exposing a specific range while ascertaining through measurement the three-dimensional coordinates in the specific range of the wafer by using the sensors, adjustment results are obtained by using the adjustment values for the individual sensors in processing the three-dimensional coordinates in the specific range having been measured by the sensors. According to the present invention in which the wafer is leveled based upon the adjustment results, the wafer is not allowed to become tilted to an excessive degree even if a sensor detects an indentation or a projection at the wafer surface during the exposure operation. Consequently, the range over which the defocusing of the exposing light attributable to the presence of a raised area on the wafer manifests can be minimized.